Novel fws dc-ac grid connected inverter

ABSTRACT

A new class of DC-AC inverter comprises a buck or two buck converters and two or four low frequency switches, and it achieves ultra-high efficiency, reactive power flow capability, small size and low cost in grid-connected applications.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. § 119(e) ofProvisional Patent Application No. 62/757,168, filed Nov. 8, 2018, theentire disclosure of which is hereby incorporated by reference.

RELATED ART

The present disclosure relates to a DC-AC power inverter, particularlyfor grid-connected applications. In the past decade, some new powerconversion topologies were proposed for grid-connected applications toachieve higher efficiency, lower cost and smaller footprint. Thefollowing publications are just some of the latest proposed topologiesin this endeavor:

-   1. S. Dutta and K. Chatterjee, “A Buck and Boost Based Grid    Connected PV Inverter Maximizing Power Yield From Two PV Arrays in    Mismatched Environmental Conditions,” in IEEE Transactions on    Industrial Electronics, vol. 65, no. 7, pp. 5561-5571, July 2018.-   2. S. Strache, R. Wunderlich and S. Heinen, “A Comprehensive,    Quantitative Comparison of Inverter Architectures for Various PV    Systems, PV Cells, and Irradiance Profiles,” in IEEE Transactions on    Sustainable Energy, vol. 5, no. 3, pp. 813-822, July 2014.-   3. L. Zhou, F. Gao and T. Xu, “A Family of Neutral-Point-Clamped    Circuits of Single-Phase PV Inverters: Generalized Principle and    Implementation,” in IEEE Transactions on Power Electronics, vol. 32,    no. 6, pp. 4307-4319, June 2017.-   4. J. F. Ardashir, M. Sabahi, S. H. Hosseini, F. Blaabjerg, E.    Babaei and G. B. Gharehpetian, “A Single-Phase Transformerless    Inverter With Charge Pump Circuit Concept for Grid-Tied PV    Applications,” in IEEE Transactions on Industrial Electronics, vol.    64, no. 7, pp. 5403-5415, July 2017.-   5. S. Saridakis, E. Koutroulis and F. Blaabjerg, “Optimization of    SiC-Based H5 and Conergy-NPC Transformerless PV Inverters,” in IEEE    Journal of Emerging and Selected Topics in Power Electronics, vol.    3, no. 2, pp. 555-567, June 2015.-   6. W. Li, Y. Gu, H. Luo, W. Cui, X. He and C. Xia, “Topology Review    and Derivation Methodology of Single-Phase Transformerless    Photovoltaic Inverters for Leakage Current Suppression,” in IEEE    Transactions on Industrial Electronics, vol. 62, no. 7, pp.    4537-4551, July 2015.-   7. Y. Zhou, W. Huang, P. Zhao and J. Zhao, “A Transformerless    Grid-Connected Photovoltaic System Based on the Coupled Inductor    Single-Stage Boost Three-Phase Inverter,” in IEEE Transactions on    Power Electronics, vol. 29, no. 3, pp. 1041-1046, March 2014.-   8. L. Zhang, K. Sun, Y. Xing and M. Xing, “H6 Transformerless    Full-Bridge PV Grid-Tied Inverters,” in IEEE Transactions on Power    Electronics, vol. 29, no. 3, pp. 1229-1238, March 2014.

All of the above-listed publications are hereby incorporated byreference in their respective entireties.

In the meantime, a wide range of power conversion topologies can also befound in a number of patent documents. Below is a exemplary list ofthose U.S patent documents.

Exemplary U.S. Patent Documents 8,369,113 B2 February 2013 Rodriguez8,582,331 B2 November 2013 Frisch et al. 8,971,082 B2 March 2015Rodriguez 9,071,141 B2 June 2015 Dong et al. 9,093,897 B1 June 2015 Wenget al. 9,148,072 B2 September 2015 Ueki et al. 9,318,974 B2 April 2016Yoscovich et al. 9,413,268 B2 August 2016 Fu et. al 9,584,044 B2February 2017 Zhou et al. 9,627,995 B2 April 2017 Ayai 9,641,098 B2 May2017 Fu et al. 9,692,321 B2 June 2017 Hu et al. 9,806,529 B2 October2017 Fu 9,806,637 B2 October 2017 Fu 9,812,985 B2 November 2017Rodriguez 9,831,794 B2 November 2017 Rodriguez 9,866,147 B2 January 2018Kidera et al. 9,871,436 B1 January 2018 Jiao et al. 9,941,813 B2 April2018 Yoscovich 10,008,858 B2 June 2018 Garrity 10,033,292 B2 July 2018Rodriguez

All of the above-listed exemplary U.S. patent documents are herebyincorporated by reference in their respective entireties.

Prevailing power conversion topologies, including, e.g., those discussedin, e.g., U.S. Pat. Nos. 8,369,113, 9,941,813, 10,008,858, and10,033,292, which are just some of the examples, in Applicant's view,are still in need of improvement in terms of performance. First, manyprevailing inverter topologies have limitations in regards to reducingswitching losses, thereby causing each to have output inductor L andpassive components that are large in size.

In this aspect, there are a few classes of grid-connected invertertopologies, namely, isolated and none-isolated inverters, mid-frequency(MF) (10 KHz-30 KHz), high frequency (HF) (above 100 KHz) or HF with lowfrequency (LF) switches inverter topologies. FIGS. 2B, 2C, 2E, and 2Hillustrate a few MF inverters. The ability of MF inverters as a whole toreduce switching losses is limited due to its MF switching frequencies.However, due to its limitations in reducing switching losses, such an MFinverter's output inductor L and passive components are relatively largein size, which is undesirable. FIGS. 2F and 2G are MF inverters with LFswitches. Those inverter topologies are still limited in terms ofcontrolling its switching losses with its MF switching frequency. As aresult, such an inverter topology (of those MF inverters with LFswitches) still sees its output inductor L and passive components largein size.

Second, many prevailing inverter topologies have limitations in regardsto having reactive power flow capability. For those grid-connectedinverter topologies, achieving reactive power flow while connecting thegrid is, however, desirable. Inverters with the topologies illustratedin FIGS. 2A, 2D and 2I, can be operated in HF with LF switches. As aresult, for such an inverter, its output inductor L can be smaller andhigher efficiency can be achieved, thus being relatively okay with theabove-discussed first aspect. However, those topologies do not havereactive power flow capability, therefore limiting them to work at unitypower generation only.

Reactive power flow capability is seen as a very important feature fortoday's inverter technology. FIG. lE shows three configurations of powerstage cells. Configuration (A) is unidirectional cell, whoseconfiguration is used in the topologies illustrated in FIGS. 2D and 2I.A skilled artisan readily appreciates and recognizes that thisconfiguration can only supports unidirectional current flow, thus beingincapable of having reactive power flow.

Continuing with the discussion on FIG. 1E, configuration (B) of a powerstage cell is a conventional half bridge configuration, which supportsbidirectional current flow that may include reactive power flow. It isworth noting that this configuration is used in almost all otherconventional topologies. Although this bidirectional configuration iseasy to be applied in IGBT type's inverter circuit, the associatedoperation switching frequency is limited. For a modern powersemiconductor device such as a MOSFET, higher switching operationalfrequency is possible. However, high voltage devices have slow recoverytimes on a body diode of such a semiconductor device, resulting in aninverter suffering from excess switching losses that otherwise can beavoided or reduced. As result, a MOSFET operating in configure (B) has apotential problem of being conducted in shoot-through, causing theinverter to break down.

Accordingly, there is a need to improve the above-discussed undesirableproblems existed in the conventional power conversion topologies.

BRIEF SUMMARY

In one aspect, the presently disclosed novel conversation topologies, asgenerally illustrated in FIG. 1(A), comprises one or two buckconverter(s). Under the presently disclosed conversion topologies, softswitching can be readily achieved, which is in contrast to H bridge orhalf bridge configurations. During each half cycle of a conductionperiod, there are two switches in the respective conduction path, whichis in contrast to other topologies (where there might have been three ormore switches connected in series). As a result, a better efficiency canbe achieved, thereby improving on the size of the associated outputinductor L and passive components.

In another aspect, under the presently disclosed conversion topologies,each set of output terminals receives low-frequency half wave signals.Thus, undesirable high frequency leakage current is minimized.

In yet another aspect, under the presently disclosed configuration (C)illustrated in FIG. 1E, the involved switches are not connected inseries as of the same phase leg. Thus, the slow body diode does notconduct current, thereby resulting in the power stage to not have ashoot-through issue, while having the capability of reactive power flow.On the other hand, the coupled inductor and the switch T1 cause thepower stage to work equivalent to a half bridge power stage, which isbidirectional. Since there are no two power devices are connected inseries, the body diode reverse recovery issue can be avoided. The switchT1 works in ZVS (zero voltage switching) mode, resulting in switchingloss being small. This then also improves the aspect of the size of theassociated output inductor L and passive components.

The above summary contains simplifications, generalizations andomissions of detail and is not intended as a comprehensive descriptionof the claimed subject matter but, rather, is intended to provide abrief overview of some of the functionality associated therewith. Othersystems, methods, functionality, features and advantages of the claimedsubject matter will be or will become apparent to one with skill in theart upon examination of the following figures and detailed writtendescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The description of the illustrative embodiments can be read inconjunction with the accompanying figures. It will be appreciated thatfor simplicity and clarity of illustration, elements illustrated in thefigures, unless expressly specified, have not necessarily been drawn toscale. Also, any text and/or any numerical data (numbers) appeared onany drawing figures is provided to illustrate an exemplary embodiment orimplementation, and thus is provided for the purpose of illustration andnot for the purpose of limitation. For example, the dimensions of someof the elements may be exaggerated relative to other elements.Embodiments incorporating teachings of the present disclosure are shownand described with respect to the figures presented herein, in which:

FIG. 1A illustrates general form of the presently disclosed DC-ACinverter.

FIGS. 1B and 1C show exemplary choices of the buck converter shown inFIG. 1A.

FIG. 1D illustrates an example of the presently disclosed DC-AC Inverterwith one buck converter and four low frequency switches.

FIG. 1E illustrates comparisons among unidirectional and bidirectionalcell configurations.

FIGS. 2A-2I illustrate respective conversion topologies in the relatedart.

FIG. 3A illustrates an example of the presently disclosed DC-AC Inverterwith two buck converters and two low frequency switches.

FIGS. 3B-C depict exemplary timing diagrams in connection with themodulation strategy of the presently disclosed DC-AC converter for FIG.3A.

FIGS. 3D-E illustrate exemplary timing diagrams in connection withmodulating timing for the presently disclosed DC-AC converter for FIG.3A.

FIGS. 4A-H illustrate four respective operation modes for the presentlydisclosed DC-AC converter for FIG. 3A, including exemplary respectivetiming diagrams associated with four operation modes.

FIG. 4I illustrates an example of the presently disclosed DC-AC Inverterreceiving reactive power flow from a large inductor series or largecapacitor parallel with Vac.

FIGS. 4J and 4K illustrate exemplary respective timing diagrams inconnection with how the presently disclosed DC-AC Inverter handlesreactive power flow from an exemplary inductive load and an exemplarycapacitive load.

DETAILED DESCRIPTION

In the following detailed description of exemplary embodiments of thedisclosure in this section, specific exemplary embodiments in which thedisclosure may be practiced are described in sufficient detail to enablethose skilled in the art to practice the disclosed embodiments. However,it is to be understood that the specific details presented need not beutilized to practice embodiments of the present disclosure. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present disclosure is defined bythe appended claims and equivalents thereof.

References within the specification to “one embodiment,” “anembodiment,” “embodiments”, or “one or more embodiments” are intended toindicate that a particular feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present disclosure. The appearance of such phrases invarious places within the specification are not necessarily allreferring to the same embodiment, nor are separate or alternativeembodiments mutually exclusive of other embodiments. Further, variousfeatures are described which may be exhibited by some embodiments andnot by others. Similarly, various requirements are described which maybe requirements for some embodiments but not other embodiments.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. Moreover, the use of the terms first, second, etc. do notdenote any order or importance, but rather the terms first, second, etc.are used to distinguish one element from another.

Those of ordinary skill in the art will appreciate that the componentsand basic configuration depicted in the following figures may vary.Other similar or equivalent components may be used in addition to or inplace of the components depicted. A depicted example is not meant toimply limitations with respect to the presently described one or moreembodiments and/or the general disclosure.

The presently disclosed DC-AC converter is illustrated in general formin FIG. 1A, comprising two buck converters and two low frequencyswitches. Referring to FIG. 1A, the DC source can be photovoltaicarrays, batteries, fuel cells or others. The AC source can be utilitygrid, single-phase electric motors or others. Each front converter canbe any unidirectional converter that can generate half sinusoidalwaveform at the frequency of the connected AC source, including but notlimited to a classic unidirectional buck converter (shown FIG. 1B) and athree-level bidirectional buck converter (shown in FIG. 1C) or any otherkind of buck converter. The two converters can be identical, orcombinations of any buck converters.

FIG. 3A shows a presently disclosed DC-AC converter with classic buckconverter configured with bidirectional cell as an example fordescription. One possible modulation strategy is depicted in FIGS. 3B-E,together with AC voltage and current waveforms. Four possible operationmodes of the presently disclosed inverter in unity power generation withthe described modulation strategy are respectively demonstrated in FIGS.4A-H.

For reactive power generation mode in connection with the presentlydisclosed topologies, FIGS. 4I-K provide an exemplary configurationdiagram and examples of reactive power flow associated with theexemplary configuration, respectively.

Mode 1

Referring to FIGS. 4A-B and FIGS. 3B-C, during this mode, T3 operates inPWM in generating an AC waveform using known general configurations thatinvolve one or more suitable inductors and one or more suitablecapacitors. T2 works as a low frequency switch. The converter generateshalf wave AC at point “A”, while T1 and T4 are off. Accordingly, powertransfers from the DC source to inductors L1 to output capacitor C3 andthe AC source. As a result, half wave of the AC sinusoidal is generatedcrosses AC source. As indicated in the timing diagrams, T5, which workssimultaneously with T3 and is coupled to L3, conducts the freewheelingcurrent through forward-biased D1 (otherwise reversed-biased during T3ON periods) during T3 OFF periods.

Mode 2

Referring to FIGS. 4C-D and FIGS. 3B-C, during this mode, which can alsobe appreciated as a specific sub-mode during Mode 1, T2 and D1 are ON,while T1, T3 and T4 are OFF and T5 is ON. Power transfers from theinductors L1, L3. capacitor C3 to the AC source. D1 keeps thefreewheeling current flow of the inductor L1 due to T5 being on, withinductor L3, which is coupled to T5, engaging T5 to connect thefreewheeling current as well.

Mode 3

Referring to FIGS. 4E-F and FIG. 3E, this mode mirrors Mode 1, exceptfor generating the other half wave of the AC sinusoidal. Thus, as askilled artisan readily appreciate, the operation of Mode 3 correspondswith the operation of Mode 1, except for that the other low-frequency T1is ON, T4 operates in PWM, while the corresponding switches of T1 andT4, namely, T2 and T3, are off. Thus, power transfers from the DC sourceto inductor L2 (which corresponds to L1) to output capacitor C4 (whichcorresponds to C3) and the AC source. As a result, the other half waveof the AC sinusoidal is generated crosses AC source. As indicated in thetiming diagrams, T6 (which corresponds with T5), which workssimultaneously with T4 and is coupled to L4, conducts the freewheelingcurrent through forward-biased D2 (otherwise reversed-biased during T4ON periods) during T4 OFF periods.

Mode 4

Referring to FIGS. 4G-H and FIG. 3E, this mode mirrors Mode 2, exceptfor generating the other half wave of the AC sinusoidal. Thus, Mode 4can also be appreciated as a specific sub-mode during Mode 3.Accordingly, during this mode, T1 and D2 are ON, while T2, T3 and T4 areOFF and T6 is ON. Power transfers from the inductors L2, L4. capacitorC4 to the AC source. D2 keeps the freewheeling current flow of theinductor L2 due to T6 being on, with inductor L4, which is coupled toT6, engaging T6 to connect the freewheeling current as well.

Reactive Power Flow Mode

Referring to FIGS. 4I, 4J and 4K, during this mode, load (AC source) isreactive, with either inductive load (lagging) or capacitive load(leading). As indicated in FIGS. 4J and 4K, the output current is out ofphase with the output voltage, the coupled inductor L1 and L3, L2 and L4engage the freewheeling current flow during the intervals when T5 and T4are on. And with the didoes D1 and D2 of the combination conducting thecurrent, the bidirectional current flow is established. See Iac1 andIac2 in FIGS. 4J and 4K. Those currents travel in two directions,showing the bidirectional current flow.

In summary, during the positive sinusoidal cycle (ν_(ac)>0), T1 remainsoff and T2 remains on. T3, D1 and T5 turn on and off in a complementaryway to generate required current i_(ac1), whereas T4 and D2 remain off.For the negative sinusoidal cycle (ν_(ac)<0), T1 remains on and T2remains off. T4, D2 and T6 turn on and off in a complementary way togenerate required current i_(ac2), whereas T3 and D1 remain off.

While the disclosure has been described with reference to one or moreexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the disclosure. Inaddition, many modifications may be made to adapt a particular system,device or component thereof to the teachings of the disclosure withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the disclosure not be limited to the particular embodimentsdisclosed for carrying out this disclosure.

What is claimed is:
 1. A bidirectional power cell, comprising: auni-directional buck converter having an output terminal, the buckconverter having a set of one or more modulated high-frequency (HF)switches configured to conduct, in a first single direction towards theoutput terminal, a switch current on a switch current path having oneend at the output terminal, the buck converter having a set of one ormore freewheeling means configured to conduct, in a second singledirection towards the output terminal, a freewheeling current on afreewheeling current path having one end at the output terminal, thebuck converter configured, with a use of the set of one or moremodulated HF switches and the set of one or more freewheeling means, togenerate and output, from the output terminal, only a uni-directionalmodulated HF AC current flowing in a single outgoing direction; areactive power flow enabler (RPFE) unit having an input terminal, theRPFE unit coupled, at the input terminal thereof, to the buck converterthrough the output terminal thereof and configured to receive theuni-directional HF AC current of the buck converter, the RPFE unithaving a bidirectional terminal and configured, at the bidirectionalterminal, to both receive an incoming reactive current from an externalload or AC source and output an outgoing current of the power cell, theRPFE unit configured, during freewheeling periods of the buck converter,to receive, from the bidirectional terminal, the incoming reactivecurrent and conduct the incoming reactive current so as to supply,through the freewheeling current path of the buck converter, thefreewheeling current of the buck converter, the RPFE unit configured toperform a low-pass filtering operation on the received uni-directionalHF output AC current so as to generate and output, from thebidirectional terminal, an outgoing low-frequency (LF) AC current as theoutgoing current of the bidirectional power cell; and wherein each HFswitch of the set of one or more HF switches of the buck converter isrespectively modulated in accord with a pre-defined waveform of theout-going LF AC current of the RPFE unit.
 2. The bidirectional powercell of claim 1, wherein the RPFE unit comprises an HF switch configuredto be oppositely biased on and off with respect to at least one HFswitch of the set of one or more HF switches of the buck converter. 3.The bidirectional power cell of claim 1, wherein the RPFE unit comprisesa coupled inductor having a first component inductor and a secondcomponent inductor, the first and second component inductors connectedin series with one another through the bidirectional terminal of theRPFE unit as a common terminal of the first and second componentinductors.
 4. The bidirectional power cell of claim 3, wherein the RPFEunit comprises an HF switch configured to be oppositely biased on andoff with respect to at least one HF switch of the set of one or more HFswitches of the buck converter, the coupled inductor of the RPFE unitconfigured to have a first terminal coupled to the output terminal ofthe buck converter and have a second terminal coupled to the HF switchof the RPFE unit.
 5. The bidirectional power cell of claim 4, whereinthe buck converter is configured to have a first input terminal, thesecond terminal of the coupled inductor of the RPFE unit coupled to adiode coupled to the first input terminal of the buck converter andreverse biased with regard to a the first input terminal of the buckconverter.